1. Field of the Invention
This invention generally relates to the fabrication of integrated circuit (IC) devices, and more particularly, to a solar cell and associated fabrication method for forming a silicon oxide-nitride-carbide (SiOxNyCz) thin film with embedded nanocrystalline semiconductor particles, using a high-density plasma-enhanced chemical vapor deposition process.
2. Description of the Related Art
The unique structural, electrical, and optical properties of nanocrystalline silicon (Si) have attracted interest for their use in optoelectronic and integrated memory devices. Silicon is the material of choice for the fabrication of optoelectronic devices because of well-developed processing technology. However, the indirect band-gap makes it an inefficient material for optoelectronic devices. Over the years, various R&D efforts have focused on tailoring the optical function of Si to realize Si-based optoelectronics. The achievement of efficient room temperature light emission from the crystalline silicon is a major step towards the achievement of fully Si-based optoelectronics.
The fabrication of stable and reliable optoelectronic devices requires Si nanocrystals with high photoluminescence (PL) and electroluminescence (EL) quantum efficiency. One approach that is being actively pursued for integrated optoelectronic devices is the fabrication of SiOx (x≦2) thin films with embedded Si nanocrystals. The luminescence due to recombination of the electron-hole pairs confined in Si nanocrystals depends strongly on the nanocrystal size. The electrical and optical properties of the nanocrystalline Si embedded SiOx thin films depend on the size, concentration, and distribution of the Si nanocrystals. Various thin-film deposition techniques such as sputtering and plasma-enhanced chemical vapor deposition (PECVD), employing capacitively-coupled plasma source, are being investigated for the fabrication of stable and reliable nanocrystalline Si thin films.
However, conventional PECVD and sputtering techniques have the limitations of low plasma density, inefficient power coupling to the plasma, low ion/neutral ratio, and uncontrolled bulk, and interface damage due to high ion bombardment energy. Therefore, the oxide films formed from a conventional capacitively-coupled plasma (CCP) generated plasma may create reliability issues due to the high bombardment energy of the impinging ionic species. It is important to control or minimize any plasma-induced bulk or interface damage. However, it is not possible to control the ion energy using radio frequency (RF) of CCP generated plasma. Any attempt to enhance the reaction kinetics by increasing the applied power results in increased bombardment of the deposited film, which creates a poor quality films with a high defect concentration. Additionally, the low plasma density associated with these types of sources (˜1×108-109 cm−3) leads to limited reaction possibilities in the plasma and on the film surface, inefficient generation of active radicals for enhanced process kinetics, inefficient oxidation, and reduction of impurities at low thermal budgets, which limits their usefulness in the fabrication of low-temperature electronic devices.
A deposition process that offers a more extended processing range and enhanced plasma characteristics than conventional plasma-based techniques, such as sputtering, PECVD, etc., is required to generate and control the particle size for PL/EL based device development. A process that can enhance plasma density and minimize plasma bombardment will ensure the growth of high quality films without plasma-induced microstructural damage. A process that can offer the possibility of controlling the interface and bulk quality of the films independently will enable the fabrication of high performance and high reliability electronic devices. A plasma process that can efficiently generate the active plasma species, radicals and ions, will enable noble thin film development with controlled process and property control.
For the fabrication of high quality silicon based films, the oxidation of the grown film is also critical. A process that can generate active oxygen radicals at high concentration will ensure effective passivation of the Si nanoparticles in the oxide matrix surrounding it. A plasma process that can minimize plasma-induced damage will enable the formation of a high quality interface that is critical for the fabrication of high quality devices. Low thermal budget efficient oxidation and hydrogenation processes are critical and will be significant for the processing of high quality optoelectronic devices. The higher temperature thermal processes can interfere with the other device layers and it is not suitable in terms of efficiency and thermal budget, due to the lower reactivity of the thermally activated species. Additionally, a plasma process which can provide a more complete solution and capability in terms of growth/deposition of novel film structures, oxidation, hydrogenation, particle size creation and control, and independent control of plasma density and ion energy, and large area processing is desired for the development of high performance optoelectronic devices. Also, it is important to correlate the plasma process with the thin film properties as the various plasma parameters dictate the thin film properties and the desired film quality depends on the target application. Some of the key plasma and thin-film characteristics that depend on the target application are deposition rate, temperature, thermal budget, density, microstructure, interface quality, impurities, plasma-induced damage, state of the plasma generated active species (radicals/ions), plasma potential, process and system scaling, and electrical quality and reliability. A correlation among these parameters is critical to evaluate the film quality as the process map will dictate the film quality for the target application. It may not be possible to learn or develop thin-films by just extending the processes developed in low density plasma or other high density plasma systems, as the plasma energy, composition (radical to ions), plasma potential, electron temperature, and thermal conditions correlate differently depending on the process map.
Low temperatures are generally desirable in liquid crystal display (LCD) manufacture, where large-scale devices are formed on transparent glass, quartz, or plastic substrate. These transparent substrates can be damaged when exposed to temperatures exceeding 650 degrees C. To address this temperature issue, low-temperature Si oxidation processes have been developed. These processes use a high-density plasma source such as an inductively coupled plasma (ICP) source, and are able to form Si oxide with a quality comparable to 1200 degree C. thermal oxidation methods.
The performance of the bulk or thin film Si solar cells strongly depends of the intrinsic properties of the constituent layers and the interfacial characteristics of the integrated structure. Some of the key parameters defining the solar cell efficiency are the absorption of the solar spectrum, light coupling into cell, carrier generation/recombination and transport across various interfaces, stability, and reliability. The optical bandgap is one of the key material properties that determines the absorption of the incident light and transport of the carrier across the interfaces in integrated devices. Extensive research is being conducted to find a suitable window material for thin film or bulk Si cells exhibiting wide bandgap, high electrical conductivity, and good interfacial characteristics. One of the key focus areas is the development of single layers serving the dual role of window and electrical contacts in thin films or as emitter in bulk Si solar cells. Typically, the optical properties of the materials are not tunable over a wide range without sacrificing the balance among the various desired properties for a particular application. Generally, a complete solution requires the use of multilayer structures involving different material systems to accomplish the desired device performance.
It would be advantageous if a low-temperature process existed for the fabrication of silicon based thin-films with embedded semiconductor nanocrystals, to be used in solar cells as a photon absorbing layer, window layer, and electrode layer.